(1) Field of the Invention
The present invention generally relates to a thin film multi-layer structure and a method for manufacturing the same, and more particularly to a thin film multi-layer structure having a plurality of metal layers and insulating layers made of polyimide used as interlayer insulation layers and a surface protection layer.
(2) Description of the Related Art
In a thin film multi-layer wiring board, interlayer insulation layers and a surface protection are made of polyimide. In order to prevent the board from being bent by residual stress generated in the interlayer insulation layers and the surface protection layer, the polyimide having a low thermal expansion coefficient is used for the interlayer insulation layers and the surface protection layer. The polyimide having a low thermal expansion coefficient is referred to as a low thermal expansion type polyimide.
FIG. 1 shows a structure of a conventional thin film multi-layer wiring board formed of a plurality of metal layers and interlayer insulation layers and a surface protection layer which are made of polyimide. Referring to FIG. 1, a plurality of thin film layers made of metal such as Cu are stacked on a substrate 1 made of AlN by using a sputtering process, a plating process and an etching process.
The metal thin film layers and the interlayer insulation layers and the surface protection layer made of polyimide are formed as follows.
Ti, Cu and Cr are successively spattered on the surface of the substrate 1 so that a thin film layer 2 is formed on the substrate 1. The thin film layer 2 is shaped in a predetermined pattern by the etching process. An interlayer insulation layer 3 made of polyimide is formed on the thin film layer 2. After this, Cr, Cu and Cr are successively sputtered on the interlayer insulation layer 3 and a plating resist is formed on the stacked structure of Cr, Cu and Cr. The Cr layer is then etched so that the Cu layer is exposed. After this, a Cu plating process and a panel etching process are applied to the exposed Cu layer so that a predetermined shaped thin film layer 4 is formed.
After this, the above procedure is repeated so that an interlayer insulation layer 5, a thin film layer 6, an interlayer insulation layer 7 and a thin film layer 8 are formed. The surface of the thin film layer 8 is provided with a thin film layer made of Au. Last, a surface protection layer 9 is formed on the surface of the stacked structure formed as described above.
In the above process, to form each of the interlayer insulation layers 3, 5 and 7, photosensitive polyimide is used as the low thermal expansion type polyimide. Lower layer portions 3A, 5A and 7A and upper layer portions 3B, 5B and 7B are separately formed by using exposure and drawing techniques. In FIG. 1, a boundary line between each of the lower layer portions 3A, 5A and 7A and a corresponding one of the upper layer portion 3B, 5B and 7B is shown by a dotted line. Every time one of the low layer portions 3A, 5A and 7A is formed, a pre-baking process is performed. Every time one of the upper layer portions 3B, 5B and 7B is formed, a hard-backing process is performed. In the pre-baking process, the stacked structure is tentatively heated at a first temperature. In the hard-baking process, the stacked structure is completely heated at a second temperature higher than the first temperature.
In the thin film multi-layer wiring board having the above structure, for example, the thin film 2 forms a ground electrode layer, the thin film layer 4 forms a power electrode layer, the thin film layer 6 forms a signal electrode layer and the thin film layer 8 forms a surface layer of the stacked structure. I/O (Input/Output) pins used to relay signals between an external unit and this wiring board are fixed on the thin film layer 8 (the surface layer) by solder. Each of the I/O pins 10 has a core made of Cu, a Ni layer formed on the surface of the core and an Al layer formed on the Ni layer. The diameter and length of each of the I/O pins 10 are respectively equal to about 0.2 mm and 4 mm.
In the thin film multi-layer wiring board having the above structure, a supporting structure (the surface protection layer 9 and/or the interlayer insulation layers) supporting the I/O pins 10 must have an enough strength to be against a force applied to each of the I/O pins 10. That is, although each of the I/O pins 10 which is pushed by a predetermined force in a direction, as shown by an arrow in FIG. 1, parallel to the surface of the wiring board may be broken or bent, it is required that the layers included in the supporting structure of each of the I/O pins 10 are not broken.
In the conventional thin film multi-layer wiring board, the low expansion type polyimide is used as material of which all the interlayer insulation layers and the surface layer are made, as has been described above. On the other hand, in general, the low expansion type polyimide has a large Young's modulus as shown in FIG. 2. Thus, after the hard-baking process is performed for the stacked structure including the layers made of the low expansion type polyimide, the layers are made be friable. As a result, there is a case where the layers included in the supporting structure of an I/O pin are broken when the I/O pin is provided with a force in a direction parallel to the surface of this wiring board before the I/O pin is broken or bent.